^.36 ORGANIC REMAINS. 



and seldom exceeds an inch in diameter. The ears are of a moderate 

 size, nearly equal. As it abounds about Rosebury Topping, we may 

 name it p. Roseburiensis. A similar shell from the same strata, inferior 

 in size, but with about fourteen ribs, approaches the common shape of 

 a plagiostoma, and may therefore be called p. plagiostomus. 



No. 7, similar in shape, is from the oolite. It has seven or eight 

 i'ibs, of which those in the middle are vastly larger than those on 

 either side. If it is not the ]). Jlavicmis, we might name it p. inequi- 

 costatus. 



OsTREA. Oyster. This is another shell abundant in our strata; 

 and were we, with Linnaeus, to include the scallops under this genus, 

 the family would be numerous in the extreme. The fossil oysters 

 occur in more of our beds than the pectens ; for we find them in the 

 ironstone and sandstone bands of the alum shale, in the alum shale 

 itself, in the dogger, in the blue limestone, in the calcareous sandstone 

 and ironstone of Scarborough and Filey, in the upper and second 

 shale, in the oolite, and in the chalk. 



The o. edulis, or common oyster, is found in all its varieties ; large, 

 and small; rough, and smooth; round, and irregular; water-worn, and 

 entire ; in single valves, and in double. The Staiths beds include 

 large seams consisting almost entirely of oysters, of various kinds, 

 and in various states. Some of the shells are of a handsome circular 

 form, others of a deltoid shape ; some are large and thick, others 

 small and thin. Very large oysters, from six to nine inches or 

 upwai'ds in length, are occasionally met with in the higher iron- 

 stone beds, near Scarborough and Filey; as also in the oolite, and in 

 the upper shale. Several of the shells alluded to may be distinct 

 species, rather than varieties of o. edulis. At any rate, we find several 

 other species of this genus, a few of which will require to be noticed. 



Fig. 14, PI. IX, represents a handsome plaited oyster, occurring 

 in the calcareo-ferruginous sandstone of Filey, and of Scarborough. 



